Owls are well known for being nocturnal, although some owls are active during the day.

Nocturnality is an animal behavior characterized by activity during the night and sleeping during the day. The common adjective is "nocturnal".

Nocturnal creatures generally have highly developed senses of hearing, smell, and specially adapted eyesight. Such traits can help animals such as the Helicoverpa zea moths avoid predators.[1] Some animals, such as cats and ferrets, have eyes that can adapt to both low-level and bright day levels of illumination (see metaturnal). Others, such as bushbabies and (some) bats, can function only at night. Many nocturnal creatures including tarsiers and some owls have large eyes in comparison to their body size in order to compensate for the lower light levels during the night. More specifically, these animals have been found to have a larger cornea relative to their eye size than diurnal creatures, in order to increase their visual sensitivity in the low light conditions.[2] Nocturnality helps wasps such as Apoica flavissima avoid hunting in intense sunlight.

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While it is difficult to say which came first, nocturnality or diurnality, there is a leading hypothesis out in the evolutionary biology community. Known as the "bottleneck theory", it postulates that millions of years ago in the Mesozoic era, many ancestors of modern day mammals evolved nocturnal characteristics in order to avoid contact with the numerous diurnal predators. A recent study attempts to answer the question as to why so many modern day mammals retain these nocturnal characteristics even though they are not active at night. The leading answer is that the high visual acuity that comes with diurnal characteristics isn't needed anymore due to the evolution of compensatory sensory systems, such as a heightened sense of smell and more astute auditory systems. The anomaly to this theory were anthropoids, who appeared to have the most divergence from nocturnality than all organisms examined. While most mammals didn't fall exhibit the morphological characteristics expected by a nocturnal creature, reptiles and birds fit in perfectly. A larger cornea and pupil correlated well with whether these two classes of organisms were nocturnal or not.[2]

Being active at night is a form of niche differentiation, where a species' niche is partitioned not by the amount of resources but by the amount of time (i.e. temporal division of the ecological niche). Hawks and owls can hunt the same field or meadow for the same rodents without conflict because hawks are diurnal and owls are nocturnal. This means they are not in competition for each other's prey.

Nocturnality is a form of crypsis, an adaptation to avoid or enhance predation. One of the reasons that (cathemeral) lions prefer to hunt at night is that many of their prey species (zebra, antelope, impala, wildebeest, etc.) have poor night vision. Many species of small rodents are active at night because most of the dozen or so birds of prey that hunt them are diurnal. There are many diurnal species that exhibit some nocturnal behaviors. For example, many seabirds and sea turtles only gather at breeding sites or colonies at night to reduce the risk of predation to themselves and/or their offspring.

Another reason for nocturnality is avoiding the heat of the day. This is especially true in aridbiomes like deserts, where nocturnal behavior prevents creatures from losing precious water during the hot, dry daytime. This is an adaptation that enhances osmoregulation.[3] One of the reasons that (cathemeral) lions prefer to hunt at night is to conserve water.

Many plant species native to arid biomes have adapted so that their flowers only open at night when the sun's intense heat cannot wither and destroy their moist, delicate blossoms. These flowers are pollinated by bats, another creature of the night.

In zoos, nocturnal animals are usually kept in special night-illumination enclosures to invert their normal sleep-wake cycle and to keep them active during the hours when visitors will be there to see them.

Hedgehogs and sugar gliders are just two of the many nocturnal species kept as (exotic) pets. Cats have adapted to domestication so that each individual, whether stray alley cat or pampered housecat, can change their activity level at will, becoming nocturnal or diurnal in response to their environment or the routine of their owners. Cats normally demonstrate crepuscular behavior, being most active in hunting and exploration at dusk and dawn.[4]